Abstract

Frame structures with URM infill is one common construction type in Nepal and other regions of the world. Although, full infill wall can be considered by using equivalent diagonal strut, it is challenging for structural engineers to take into account the presence of openings in the infill wall during the seismic design of buildings. Few macro models have been developed to address this problem, but they are either too complicated to use or they create anomaly due to huge variation in the results. A new macro modelling technique is introduced here for URM infill wall with opening where the infill wall is discretized with the help of rigid elements. The discretized infill zone is represented by pair of diagonal link elements and the presence of opening is considered by removing the diagonal link in the zone of opening. The proposed model can be used for nonlinear static analysis of RC/steel framed building with masonry infills in earthquake prone regions like Nepal.
Keywords: unreinforced masonry infill (URM), opening, macro modelling

1.Introduction

Masonry infill can significantly alter the strength and stiffness of the surrounding frame. But the presence of opening in the infill wall changes these properties. However, infill wall with openings are also found to provide adequate resistance against lateral loading (Kakaletsis & Karayannis, 2007; Sigmund & Penava, 2014) and therefore those walls cannot be neglected during the design of infilled frame structures.
The size and position of the openings in the infill is usually not fixed. This variability leads to significant uncertainty. Experiments have shown that openings reduce the strength and stiffness of the masonry infill and their position also influence the behavior of infilled frame (Mallick & Garg, 1971; Dawe & Seah, 1989, Kakaletsis & Karayannis, 2007; Penava, 2012). Reduction of the strength contributes to in-plane damage of infill wall and the in-plane damage can accelerate out-of-plane failure of an infill wall during seismic action. The formation and propagation of crack pattern is also dependent on the position and size of the opening (Mosalam et al., 1997).
Macro model like multiple strut model (Fig 1.a) and discrete element model (Fig 1.b) can consider openings in masonry wall but they have certain complexity and constraints. In the multiple strut model, the grid divisions to define the diagonal strut is not unique and it can be done only for the approximate frame-infill contact length. Discrete element model is a sophisticated macro model which can represent the in-plane failure mechanisms (flexure failure, diagonal shear failure and sliding shear failure) in the infill wall. However, this models involves different spring types (Pantò et al., 2017) and is implemented in FE software 3D macro. This model is not easy to adopt in commercially available software as all features may not be available. The third group of macro model is the equivalent single strut model (Fig 1.c) which involve a stiffness reduction factor (ρ) to reduce the width of the original diagonal strut to take into account the presence of opening in the infill wall. This model is easy to implement in large structures too but the problem is that there are number of authors proposing different equations to calculate reduction factor. These stiffness reduction factors are derived based on different considerations. Therefore, this model creates anomaly because of varying results. Some of the popular models in this category are Dawe and Seah (1989), Al-Chaar (2002), Asteresis et al. (2011), Decanini et al. (2012).Moreover, this model cannot consider the position of opening.
A new macro model to analyze URM infilled frame with openings under seismic action is presented here. In this model, URM infill wall is discretized with the rigid elements and discretized infill zone is represented by pair of diagonal link elements. The presence of opening is considered by removing the diagonal link in the portion where there is opening (Fig 2).

2.Numerical modelling

Masonry infill is discretized using zero mass rigid elements and each infill subcomponent is represented by pair of diagonal struts. The width of the diagonal strut is calculated according to Smith et al. (1969) and Mainstone (1974). Multilinear plastic link element available in SAP 2000 is used to model the equivalent diagonal strut. The nonlinear property of the diagonal link in compression is defined by using trilinear force deformation relationship of Panagiotakos & Fardis (1996). The details about the macro model can be found in Pradhan (2018).
The model is calibrated with the result of the solid infill specimen from experimental study of Penava (2012). The RC frame has 2 columns (20 cm x 20 cm), one beam (20 cm x 12 cm) and a continuous foundation plate (42.5 cm x 30 cm). Fig 3 compares the capacity curve obtained after a pushover analysis for the solid infilled frame with the test and micromodel results, the model shows good agreement. The model is further validated with other specimens from the same experiment for different cases of opening in the infill wall. The load deformation curves for opening cases are shown in Fig 4.

The test specimens with openings are also analysed with some equivalent single strut models as discussed earlier. Result of the equivalent single strut models have huge variations and are shown in Fig 5. Al-Chaar (2002) fits closely for the case of door openings, yet it predicts ultimate strength less in case of window openings.

3.Conclusions

The proposed macro model is useful for the analysis of infilled frames with openings under seismic action. The model is simple and the modelling effort required is quite reasonable. The relative advantage of the model is that it can take into account the position of openings in the infill wall. This model can be useful for the design of Steel/ RC framed buildings with masonry infill in earthquake prone regions like Nepal.

References

Pradhan Bharat. Macro element modelling of unreinforced masonry infill wall with openings under seismic action, e-pub.un-weimar.de., Bauhaus-Universität Weimar, Germany. 2018

Penava, Davorin; Pradhan, Bharat; Cavaleri, Liborio; Abrahamczyk, Lars; Schwarz, Jochen; Sarhosis, Vasilis. Proračunski postupak za ocjenu potresne otpornosti konstrukcijskog sustava okvir-ziđe uzimajući u obzir veličinu, vrstu i razmještaj otvora. Proceedings of the 9th Assembly of Croatian society of Mechanics, Zagreb, 2019. 249-256